Can send mail/ can not receive

Have set up a couple of e-mail accounts to try out the mail, but I am not able to receive on any. Have tried Roundcube, Squirrelmail and Webmail. But they all give the same response.
I can login to them all and send mail to my hotmail account, but when I reply or try to send a new one back, I get a reply almost immediately postmaster delivery failure.

Using Ispconfig2.2.35 on Ubuntu 9.10

All ports for mail are open, I think, and all green in server>services

Thanks for bearing with me falko, I think I dig'd the wrong address to begin with.
I currently have 2 sites up and running, which I have dig mx.....

postrangersgolfsociety.org.uk &
berilacsshack.com

I have also had a look in my manager control panel for my dedicated server. And have set e-mails to be handled by my own server and not the server provider, as per their instructions, for one of the sites. To see if that makes any difference. Have to wait 24hrs for this to go thru.

Have included the results of dig mx my.sites.com for info anyway.

I am sure this is really simple to resolve, and I am confusing myself more than anything.

Your help is really appreciated

Its just strange that I can e-mail out, but not receive, out of the 2 accounts that I have created?